Flood mapping is increasingly used as the basis for assessing and planning flood-protection measures, and can become a sound method to enhance the preparedness of communities and businesses to extreme weather events. Flood mapping around the world uses the concept of return periods to decide how big a flood event is, but many people believe that this method is confusing and is actually outdated. I think cities, political leaders and citizens would all be better off moving to a fiscal measure for flooding, purely based on predicted flood damages since this will create a system that everyone can understand, hopefully leading to faster and more appropriate action, and greater resilience.

Return period is a statistical method used by hydrology and other sciences to establish how often an event occurs in a long dataset. So if an event of a certain size happened ten times in a 100 year dataset, it would have a return period of one in ten years. This does not mean that it will happen every ten years, just that a similar event ‘happened’ ten times in the last 100 year dataset we examined. Some of these events may have happened in the same year and then not again for 30 years.

So it is not really a generic measure of how often it happens, it is just a measure of how many times it happened in the dataset we are looking at. So whilst it is known as a measure of frequency, it does not tell us how frequently it will happen unless you refer to the original dataset.

Another confusing part of return period is that it is used to decide how big an event is, when in fact it is a measure of how ‘frequent’ an event of this size or larger could be expected to happen (in a long dataset). I also used the term event, which itself is a subjective term as we can have ‘events’ within an event and if you change the start and finish time of an event, you change its return period.

It is difficult to find another instance of this type of measure being used, which is good news as it would appear to be similar to a doctor diagnosing a wound simply by the amount of pain the patient felt (ignoring the term fatal wound).

It is therefore not surprising that many people believe that the use of return periods is confusing. There have been other arbitrary measures around such as referring to previous actual events i.e. the 2007 flood in Hull, but these are only of use in areas that have previously flooded and only of use in that locality.

I would suggest that we probably need to move to a fiscal measure for flooding that could be purely based on predicted flood damages. It is a lot easier to justify spending £1 million on a 1m high flood defence when the area behind the defence would be subject to £10 million of damages with 1m of flooding. I know we use cost benefit analysis on flood defences already, but this is based on providing protection to a certain ‘return period’. We should drop the return period and just make the financial assessments on a sliding scale of flood height costs against defence height costs.

So if it costs less to protect an area than it does to not protect it, society should protect it. However, some may think this solution is too simplistic and prefer to stick with trying to justify using the frequently misunderstood concept of return periods.

This is a modified version of a post first published on Arup Thoughts: http://thoughts.arup.com/post/details/553/a-fiscal-measure-for-flooding

For more information on ARUP, please see: http://www.arup.com/markets/water

The choices we make today will greatly influence our vulnerability to risks in the future. A greater understanding of the costs and benefits of flood resilience strategies is needed to plan and implement the best solutions for climate-resilient smart cities, now and in the future. The IWA Principles for Water Wise Cities provides the frameworks and inspiration to assist urban leaders to address these considerations and evaluate costs, and also to identify opportunities and synergies in developing and implementing a vision for resilient planning and design in their cities. Read more about the IWA Principles here.